The prevalence of multi-drug resistant bacterial pathogens has increased dramatically in recent decades, threatening our ability to treat infections. Though pathogens may develop antibiotic resistance through individual mutations, the most common means of acquiring resistance is through horizontal gene transfer (HGT), enabling pathogens to rapidly develop insensitivity to antibiotic therapy. Therefore, it is essentil to systematically characterize the many genetic reservoirs of antibiotic resistance genes (or 'resistomes') accessible to pathogens. The human gut microbiota harbor a particularly important resistome to study due to (1) easy contact and genetic exchange between commensals and pathogens, (2) historic under sampling of this community with culture-based approaches, and (3) the dynamic properties of community composition in early human life. We propose to deeply characterize the development of the intestinal resistome in the first two years of life using a powerful, culture-independent combination of functional metagenomics selections with next-generation sequencing. We seek to achieve two overarching goals. First, we will define how genetic and environmental factors (including antibiotic treatment) affect the assembly and dynamics of the infant gut resistome. Second, we will understand how the potential for mobilization of the resistome (defined by association of resistance genes with mobile genetic elements) influence the stability of this critical ecosystem. To this end, our first specific aim i to characterize resistome development in healthy infants through testing the hypothesis that antibiotic exposure, postnatal age, and shared environment and host genetics drive the abundance, diversity and dissemination of gut resistomes. Our second aim is to understand pathologic resistome development of very-low birth weight (VLBW) infants by testing the hypothesis that spectrum and duration of antibiotic therapy drive the abundance, diversity, and dissemination of the gut resistome in these characteristically low-diversity microbiotas. In both aims, we will focus on diversity, abundance, and genetic context of resistance genes in the developing microbial community. We will enhance fundamental understanding of host-associated microbial community dynamics in three significant ways: (1) Illuminating assembly and dynamics of the resistome in developing gut microbiota of infants sampled longitudinally over the first two years of life, (2) defining the role of genetic exchange in developing microbial communities using antibiotic resistance and associated mobile genetic elements as clinically-relevant and easily-assayed microbial community functions, and (3) applying technological innovations in metagenomics, next-generation sequencing, and computational biology to dramatically increase throughput and decrease costs of studying microbial community functions. Potential impacts of our study are: (1) developing a novel framework for economical, high-throughput characterization of microbial community functions, (2) providing a basis for future work to mitigate infant morbidity and mortality in the neonatal period resulting from inappropriate colonization dynamics of gut microbiota, and (3) establishing a translational evidence base for more prudent use of antibiotics.